This invention relates to the use of Fourier Transform infrared (FTIR) spectroscopy in the analysis of thin layer chromatography (TLC) samples.
The use of the diffuse reflectance mode of FTIR spectroscopy has greatly improved the effectiveness of TLC techniques. Prior to the combination of FTIR and TLC techniques, the use of TLC had lagged behind the use of high performance liquid chromatography (HPLC).
Until recently, the general use of a combined TLC/FTIR system in the analysis of samples was very clumsy and time-consuming. After the separated samples were deposited on a TLC plate, each sample was scraped off the plate (usually a silica-gel-precoated plate) and then transferred to a receptacle, such as a cup containing powdered potassium bromide (KBr), where it was submitted to diffuse reflectance FTIR spectroscopy.
The problem of sample transfer has been a deterrent in the use of TLC/FTIR. The samples cannot be successfully analyzed without removal from the TLC plate, because the silica powder on the plate is both chemically active and radiation absorbing.
A significant advance in this field, involving automatic sample transfer, was discussed in an article titled "Sample Transfer Accessory for Thin-Layer Chromatography/Fourier Transform Infrared Spectrometry" by Shafer, Griffiths and Shu-Qin, which appeared in "Analytic Chemistry, 1986, 58, pages 2708-2714. The article described a "sample transfer approach to thin-layer chromatography/Fourier transform infrared spectrometry (TCL/FT-IR) * * in which each separated component is moved simultaneously from the TLC plate to an IR-transparent substrate prior to measurement of its diffuse reflectance spectrum." As stated in the article, "more spectral information is obtained about the analytes and in significantly less time than is required for in situ measurements. The transfer is accomplished with minimal sample loss, decomposition, or contamination compared with previous processes."
In the prior art arrangement, the developed TLC plate was turned 90.degree. and placed in solvent contained in a reservoir. Using a multiplicity of vertically-extending wicks, the horizontally spaced samples were moved upwardly by the solvent and the wicks into a row of cups each containing a diffuse reflectance powder, such as a "finely-ground infrared-transmitting glass (powder) composed of germanium, antimony and selenium".
The transfer process described above is highly desirable as a TLC/FTIR sub-process. However, significant problems have been encountered in attempting to use this transfer process. Various forms of wicks have proved unsuccessful, particularly because of swelling problems, which, of course, interfere with the desired capillary upward motion of the dissolved samples. Also, imperfect sample drying at the powder-containing cups has been a particularly difficult problem to solve. If drying occurs too quickly, the full sample will not reach the diffuse reflectance cups. If drying occurs too slowly, part of the sample will be carried out above the cups by the solvent. The heating step mentioned in the cited article as a means of solvent removal has not proved to be desirable as a drying method. Yet another problem in the prior art arrangement was the tendency to cause substantial contamination due to excessive handling of elements which contacted the transfer solvent.